specify session name on perform rejoin stack

rejoin-stack.sh would not specify screen session name which will lead
unstack.sh unable to kill the screen session started by rejoin-stack.sh
after hypervisor rebooted. So specify the session name if
rejoin-stack.sh need to start a new sreen session.

fixes bug 1182806

Change-Id: I68635887aa82fa63f3667e0f090ea2836eec7b68
diff --git a/stackrc b/stackrc
index edf5a82..2ac564c 100644
--- a/stackrc
+++ b/stackrc
@@ -245,6 +245,9 @@
 # Compatibility until it's eradicated from CI
 USE_SCREEN=${SCREEN_DEV:-$USE_SCREEN}
 
+# Set default screen name
+SCREEN_NAME=${SCREEN_NAME:-stack}
+
 # Local variables:
 # mode: shell-script
 # End: